Fire doors are designed to resist a rise in temperature over a desired degree on the side of the door opposite the fire or other combustion source. Fire doors maintain a certain degree of rigidity to resist gaps or other openings from developing, which may allow flames to move around or through the door. Conventional fire doors may provide a required amount of resistance to heat and/or ballistics. However, such conventional fire doors are often quite thick to provide the required amount of resistance to heat and/or ballistics. Due to their increased thickness, conventional fire doors cannot be retrofit onto an existing door frame, such as, for example, an existing steel door frame and thus require a custom door frame in order to be properly installed in a building or other structure. This disclosure describes an improvement over these prior art technologies.